Perl 6 -p, 57 41 bytes

$!=m/\./.to;.=substr(0,$!+$/)while m:c/./

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Explanation

$!=m/\./.to;       # Store position of decimal point in $!
.=substr(0,$!+$/)  # Truncate string to length $!+$/
while m:c/./       # while there's a match for the next digit

Perl 6 -p, 57 41 bytes

$!=m/\./.to;.=substr(0,$!+$/)while m:c/./

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Explanation

$!=m/\./.to;       # Store position of decimal point in $!
.=substr(0,$!+$/)  # Truncate string to length $!+$/
while m:c/./       # while there's a match for the next digit

Perl 6, 57 44 bytes

{$!=m/\./.to;.=substr(0,$!+$/)while m:c/\d/}

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Modifies a string in-place.

Explanation

{
 $!=m/\./.to;       # Store position of decimal point in $!
 .=substr(0,$!+$/)  # Truncate string to length $!+$/
 while m:c/\d/      # while there's a match for the next digit
}

Perl 6 -p, 57 41 bytes

$!=m/\./.to;.=substr(0,$!+$/)while m:c/./

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Explanation

$!=m/\./.to;       # Store position of decimal point in $!
.=substr(0,$!+$/)  # Truncate string to length $!+$/
while m:c/./       # while there's a match for the next digit

Perl 6, 57 bytes

{S[\.<((\d)*]=[~] $0[^reduce {$^a>($^b&++$)??$b!!$a},$0]}

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Explanation

{
 S[          # replace regex match
   \.        #   literal dot
   <(        #   start match
   (\d)*     #   sequence of digits, stored in $0
 ]=          # with
 [~]         #   concatenation of
 $0[^        #   digits up to
   reduce {          # reduce digits
     $^a>($^b&++$)   # if digit and index less than acc
     ??$b            # then digit
     !!$a            # else accumulator
   },$0
 ]
}

Perl 6, 57 44 bytes

{$!=m/\./.to;.=substr(0,$!+$/)while m:c/\d/}

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Modifies a string in-place.

Explanation

{
 $!=m/\./.to;       # Store position of decimal point in $!
 .=substr(0,$!+$/)  # Truncate string to length $!+$/
 while m:c/\d/      # while there's a match for the next digit
}