This equivalent on the command line:

perl -alne 'print $F[1] if /user\@example.com/' /home/bob/db.txt
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See Command Switches.

Expanded code:

use warnings;
use strict;

open my $FH, '<', '/home/bob/db.txt' or die $!;

while (local $_ = <$FH>)
{
    chomp;
    print((split ' ')[1], "\n") if /user\@example.com/;
}
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Update: Added expanded code.