open(FH, '<', 'somefile.txt') or die $!;
my @in = <FH>;
close(FH);

my $want_username = 'myusername';
my $hit;
while( (! $hit) && (my $entry = shift @in) ) {
    my $in_user = (split /:/, $entry)[1];
    $hit = ($in_user eq $username);
}