PHP, 65 52 bytes

-13 bytes by using a function and dropping the need to handle multi-line inputs from STDIN.

function($s){return'!'.join('!',str_split($s)).'!';}

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PHP (7.4), 31 bytes

Based on Petah's comment.

fn($s)=>preg_replace(__,'!',$s)

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PHP, 52 bytes

function($s){return'!'.join('!',str_split($s)).'!';}

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PHP, 65 bytes

while(''<$l=fgets(STDIN))echo'!'.join('!',str_split($l));echo'!';

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Not sure if there is a way to get multi-line input shorter than this.

PHP, 65 52 bytes

-13 bytes by using a function and dropping the need to handle multi-line inputs from STDIN.

function($s){return'!'.join('!',str_split($s)).'!';}

Try it online!