The disadvantage of that idiom is that the scope of $line must extend outside the block where is it used. Declaring $line inside the block results in a syntax error 

use strict;

do {
    my $line = <STDIN>;
    ...
} until $line  eq ".\n";  # Syntax error
__END__
Global symbol "$line" requires explicit package name

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unlike a while(condition) {} the condition is not in the same scope as the block. It can be written as 
use strict;

INPUT:
while ( my $line = <STDIN> ) {
    last INPUT if $line eq ".\n";
    ...
}

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which will restrict $line to the while block

Update: Code updated

In reply to Re^2: do {$i++} until vs. $i++ until by hipowls
in thread do {$i++} until vs. $i++ until by jethro

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