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use strict;
use warnings;
...
STR

open my $fIn, '<', $fileStr or die "Couldn't open \$fileStr: $!\n";

# Look for the empty line between records
local  $/ = "\n\n";

while (defined (my $record = <$fIn>)) {

    my %recordData = map{split /\s+/, $_} grep {length $_} split "\n",
+ $record;
    my @solvents = grep {/^solvent\d+/} keys %recordData;
    my @fractions = grep {/^F\d+/} keys %recordData;

    my ($zeroSolvent) = grep {!$recordData{$_}} @solvents;

    
    print "${zeroSolvent}_$_ => $recordData{$_}\n" for @fractions;
}

solvent1_F101 => 3.2
solvent1_F001 => 1.2
solvent2_F101 => 7.2
solvent2_F001 => 2.2

use strict;
use warnings;
...
    
    print "${zeroSolvent}_$_ => $recordData{$_}\n" for @fractions;
}