my $pref = 'pref1'; 

open(FH, "+< $FILE") or die "Error opening $FILE: $!\n"
my @lines = <FH>; 
seek(FH,0,0); 

my $count = 0; 
foreach my $l ( @lines ) { 
    $count++; 
    # Skip lines not matching $pref
    if( $line !~ /^$pref/ ) { next; }

    my ($pref, $num) =~ /^(.*?)\t(.*?)$/; 
    $num = 1; 

    $lines[$count] = "$pref\t$num\n"; 
}

print FH @lines; 
truncate(FH, tell(FH)); 
close(FH);