#!/usr/bin/perl -w
use strict;

open F, '<', "myfile" or die "myfile: $!\n";
chomp(my @content = <F>);
close F;

# Input validation routine
my $in;
{
    print "Please enter the line number you want to retrieve (1-" . @content . "): ";
    chomp($in = <STDIN>);
    # line_numbers == array_indexes shifted by + 1
    redo unless $in =~ /^\d+$/ && $in > 0 && $in <= @content;
}

# Now you can play with your retrieved value
my ($mnum, $mname) = split / /, $content[$in - 1], 2;
print "Number: $mnum Name: $mname\n";